Metropolis was a ruined metropol that was owned by Dr. Eggman.
Eggman's World Conquest
After Dr. Eggman got ahold of a mysterious artifact known as the Phantom Ruby and successfully took over the world, Metropolis explicitly featured its identity as Eggman's headquarters and capital central for the first time. Over the course of Eggman's six months of conquest, this mega city became the heart of the world-spanning Eggman Empire. Eventually, the Resistance would take back the city from the Eggman Empire.
After the War
Metropolis was heavily damaged during the Eggman Empire-Resistance war, a war which ended with the Resistance's victory. In the aftermath, Metal Sonic's upgrades to Neo Metal Sonic were completed and he surfaced in Metropolis to aid Dr. Eggman in the final battle only to discover that he was too late. (StH: #7)
Metropolis is a large metropol that consists of countless futuristic skyscrapers, many of which carry dark spheres on top of them. Highways also wind between the top of the skyscrapers. In the present however, much of the city lies in ruins, with many of the buildings either displaying cracks or having been tipped slightly over.
- Metropolis was based on the location of the same name in the Sonic the Hedgehog video game series, where the version seen in the comic first appeared in Sonic Forces. It was unnamed in the comic, but is considered tier 2 canon by this site's canon policy, as it does not contradict already established canon from tier 1.
- As Neo Metal Sonic appeared in Metropolis after its upgrades were completed, it is possible that it was in Metropolis itself where said upgrades took place.